1. Field of the Invention
The present invention relates to a demodulator for a differential phase shift keying signal in optical fiber communication, particularly optical fiber communication using DWDM (Dense Wavelength Division Multiplexing).
2. Description of the Related Art
Recently, in order to accommodate requests for a higher speed and larger capacity of a network in accordance with rapid development of the Internet, optical fiber communication in which information is transmitted not in the form of an electric signal but in the form of an optical signal that uses an optical fiber as a transmission path has been developed and put to practical use. In such optical fiber communication, in order to realize a higher speed and a larger capacity, attention is given to DWDM (Dense Wavelength Division Multiplexing) in which plural optical signals of different wavelengths are recombined and transmitted through one optical fiber by using the property of light that “light beams of different wavelengths do not interfere with each other”.
In optical fiber communication in which DWDM is used, an optical signal which is modulated by DPSK (Differential Phase Shift Keying) or DQPSK (Differential Quadrature Phase Shift Keying) is mainly transmitted, and a received optical signal is demodulated by a demodulator comprising a delay interferometer.
For example, JP-T-2004-516743 discloses a demodulator which demodulates a DQPSK-modulated optical signal in optical fiber communication in which DWDM is used. FIG. 4 is a block diagram of the configuration of the demodulator.
As shown in FIG. 4, the related demodulator 60 is configured by: a first branch path 61 and a second branch path 62 which are formed by an optical fiber; a first Mach-Zehnder interferometer 63 and a second Mach-Zehnder interferometer 64 which are of the optical waveguide type; a first balanced optical detector 65; and a second balanced optical detector 66.
The first optical fiber 61 splits a DQPSK-modulated optical signal (hereinafter, referred to as DQPSK optical signal) transmitted from an optical fiber F, and transmits a split signal to the first Mach-Zehnder interferometer 63. The second branch path 62 splits the DQPSK optical signal transmitted from the optical fiber F, and transmits a split signal to the second Mach-Zehnder interferometer 64.
The first Mach-Zehnder interferometer 63 is configured by a first optical waveguide 63a, a second optical waveguide 63b, a third optical waveguide 63d, and a fourth optical waveguide 63e. The first optical waveguide 63a has an optical path length which is longer by ΔL1 than that of the second optical waveguide 63b, the DQPSK optical signal transmitted from the first optical fiber 61 is split, and a split signal is transmitted to the third optical waveguide 63d. The second optical waveguide 63b has a predetermined optical path length, the DQPSK optical signal transmitted from the first optical fiber 61 is split, and a split signal transmitted to the fourth optical waveguide 63e. 
The optical path length difference ΔL1 between the first optical waveguide 63a and the second optical waveguide 63b is set so that the DQPSK optical signal transmitted through the first optical waveguide 63a has a delay time which is equal to one period of the modulation rate, i.e., a symbol period with respect to that the DQPSK optical signal transmitted through the second optical waveguide 63b. A predetermined voltage is applied by a voltage applying apparatus (not shown) to give a phase shift of π/4 to the DQPSK optical signal transmitted through the second optical waveguide 63b. 
The DQPSK optical signals transmitted from the first optical waveguide 63a and the second optical waveguide 63b are recombined, and transmitted as a first recombined optical signal to the third optical waveguide 63d and the fourth optical waveguide 63e. The third optical waveguide 63d transmits the first recombined optical signal transmitted from the first optical waveguide 63a, and emits the signal toward a first light receiving element 65a of the first balanced optical detector 65. The fourth optical waveguide 63e transmits the first recombined optical signal transmitted from the second optical waveguide 63b, and emits the signal toward a second light receiving element 65b of the first balanced optical detector 65. The third optical waveguide 63d and the fourth optical waveguide 63e are configured so as to have the same optical path length.
The second Mach-Zehnder interferometer 64 is configured by a first optical waveguide 64a, a second optical waveguide 64b, a third optical waveguide 64d, and a fourth optical waveguide 64e. The first optical waveguide 64a has an optical path length which is longer by ΔL1 than that of the second optical waveguide 64b, the DQPSK optical signal transmitted from the second optical fiber 62 is split, and a split signal is transmitted to the third optical waveguide 64d. The second optical waveguide 64b has a predetermined optical path length, the DQPSK optical signal transmitted from the second optical fiber 62 is split, and a split signal is transmitted to the fourth optical waveguide 64e. 
Similarly in the first Mach-Zehnder interferometer 63, the optical path length difference ΔL1 between the first optical waveguide 64a and the second optical waveguide 64b is set so that the DQPSK optical signal transmitted through the first optical waveguide 64a has a delay time which is equal to a symbol period with respect to that the DQPSK optical signal transmitted through the second optical waveguide 64b. A predetermined voltage is applied by the voltage applying apparatus (not shown) to give a phase shift of −π/4 to the DQPSK optical signal transmitted through the second optical waveguide 64b. 
The DQPSK optical signals transmitted from the first optical waveguide 64a and the second optical waveguide 64b are recombined, and transmitted as a second recombined optical signal to the third optical waveguide 64d and the fourth optical waveguide 64e. The third optical waveguide 64d transmits the second recombined optical signal transmitted from the first optical waveguide 64a, and emits the signal toward a first light receiving element 66a of the second balanced optical detector 66. The fourth optical waveguide 64e transmits the second recombined optical signal transmitted from the second optical waveguide 64b, and emits the signal toward a second light receiving element 66b of the second balanced optical detector 66. The third optical waveguide 64d and the fourth optical waveguide 64e are configured so as to have the same optical path length.
The first balanced optical detector 65 comprises the first light receiving element 65a and second light receiving element 65b which output an electric signal in accordance with the light intensity of the first recombined optical signal. Electric signals output from the first and second light receiving element 65a, 65b are subjected to an balancing process to output a first demodulated signal x. The second balanced optical detector 66 comprises the first light receiving element 66a and second light receiving element 66b which output an electric signal in accordance with the light intensity of the second recombined optical signal. Electric signals output from the first and second light receiving element 66a, 66b are subjected to an balancing process to output a second demodulated signal y.
As described above, the demodulator comprises: the first Mach-Zehnder interferometer 63 having the two optical waveguides which apply the delay time that is equal to the symbol period, and the phase shift of π/4 to the DQPSK optical signal; and the second Mach-Zehnder interferometer 64 having the two optical waveguides which apply the delay time that is equal to the symbol period, and the phase shift of −π/4 to the DQPSK optical signal. Therefore, the obtained first and second demodulated signals x, y are signals indicative of a binary code.
A demodulator which demodulates a DPSK-modulated optical signal is requested to comprise one Mach-Zehnder interferometer. That is, such a demodulator is configured only by, shown in FIG. 4, the first branch path 61, the first Mach-Zehnder interferometer 63, and the first balanced optical detector 65. In this case, in the first Mach-Zehnder interferometer 63, it is not necessary to give the phase shift to the DPSK-modulated optical signal transmitted through the second optical waveguide 63b. 
As described above, in the related demodulator, a Mach-Zehnder interferometer of the optical waveguide type is used. Consequently, there arise the following problems.    (1) In order to stabilize characteristics of optical waveguides, a highly accurate temperature control is required. This makes the apparatus cost to be increased, and the apparatus size to be enlarged.    (2) Optical waveguides are easily affected by mechanical stress, and characteristics of optical waveguides are dispersed.    (3) Although not illustrated in FIG. 4, usually, a Mach-Zehnder interferometer is connected to an balanced optical detector by an optical fiber. Therefore, an optical signal transmitted from the Mach-Zehnder interferometer is delayed by the optical fiber.    (4) It is difficult to ensure the reproducibility of the ±π/4-phase shift process on a DQPSK optical signal.
Because of the problems, the related demodulator cannot perform stable and correct demodulation.